Damp at the bottom of a wall: rising damp, penetrating damp or a bridged DPC?
You may have a quote for a chemical damp course in your hand. Before you sign it, go outside and look at the ground.
Damp along the bottom of a wall is more often a bridged damp proof course than rising damp. Go outside and look at what is piled against the wall: soil, a path, a raised bed or render carried down past the damp proof course all let water step over it. Check that before anybody injects anything.
The short version
- The damp proof course has to sit clear of the ground. If soil, a path, a patio or a flowerbed is level with it or over it, it is bridged, and a spade fixes a good share of those.
- Keep two weeks of dated notes. Worse after rain points outside and above. Worse in cold still weather with black mould is condensation. Barely changing is the only pattern that leaves rising damp on the table.
- A meter reading taken on plaster is not proof of rising damp. Those meters read electrical resistance, and salts, foil-backed paper and plaster all read high.
- White powder is salts that water carried out of the masonry and left as it evaporated. Evidence that water moved, not a disease in itself.
Start outside
Go and look at the ground before you do anything else
Almost every house built since the late Victorian period has a damp proof course: a slate, bitumen or plastic layer built in a few courses up to stop ground water climbing the brickwork. It works only while it is clear of the ground, the principle Approved Document C runs on.
Let soil, paving or gravel sit level with that line and water steps around it. BRE names that as one of the ways ground moisture gets in: bridging of these defences by earth or ground works externally.
Finding it takes thirty seconds and a tape. Walk the outside of the wall that is damp inside, look for the horizontal line of a different material two or three courses up, then look at where the ground is. If you cannot see that line at all, because the path or the render runs straight past it into the soil, you have your answer.
The usual culprits: a patio or drive laid over the old one so the level went up by the thickness of a slab, a flowerbed banked against the house, render carried down past the line so the coating bridges it, and rubble left in the bottom of a cavity. Only the last is invisible from outside.
Before you start digging. Lowering a path or border is ordinary work, but do not excavate below the bottom of the foundation and do not undercut it. Gas, water and electricity often enter a house at low level exactly where you want to dig, so find your service entries first.
The list
What damp at the bottom of a wall usually turns out to be
Damp at the bottom of a wall has five realistic causes, and they are not equally likely. Government guidance splits damp four ways, into condensation, penetrating, rising and traumatic, and says condensation is the most common of them. Add a bridged damp proof course and that is the field.
| Could be | The test | What it means |
|---|---|---|
| Bridged damp proof course | Find the damp proof course line outside and measure down to the ground. | Ground level with it or over it rules bridging in. Dig it away and watch for a season. |
| Penetrating damp from above | Stand outside in heavy rain and watch the gutter, downpipe and gully. | Damp that darkens within a day of rain and dries back over a dry fortnight is external. |
| Condensation on a cold wall | Behind furniture, in a corner, in an unheated or north-facing room? Worst in winter? | Black spot mould and clamminess rather than a tide mark. Nothing to do with the ground. |
| A plumbing leak | Everything off, read the water meter, use no water for half an hour, read again. | A meter that has moved means a live supply leak. No outside work touches it. |
| Rising damp | Two weeks of notes with no rain correlation, and an even tide line. | A candidate, not a finding. Confirming it means drilled samples, not a meter on the plaster. |
Two more discriminators. Height. Ground moisture climbs a limited distance and stops, so damp at first-floor level is not rising damp whatever an instrument says. Construction. A cavity wall, most houses from the 1930s on, has an air gap between two leaves, so water must be carried across by something solid: mortar on a wall tie, debris at the cavity base, or the ground. A solid wall has no such break.
From above
Water arriving from higher up, and how to catch it in the act
The commonest single defect behind damp at the bottom of a wall is a rainwater fault that only shows in heavy rain: a gutter joint weeping onto the wall face, a downpipe split at the back where you cannot see it, a downpipe discharging onto the ground instead of a gully, a silted gully backing water against the brickwork.
Each soaks one strip of wall over and over, and each is cheap next to a chemical damp course. Look during a downpour, not after one, which is why a gutter that only overflows in heavy rain is worth understanding: overflowing in light rain too is a blockage, overflowing only in a downpour is capacity, fall or outlet.
Then look at the wall face directly behind the damp patch inside. Open mortar joints, a crack in render or a hollow-sounding patch will all pass water. On older solid walls hard cement pointing on soft brick traps water in the brick, so the choice between lime and cement mortar is a moisture decision first. Repairing joints and failed render is ordinary bricklaying and repointing work, and a smaller job than the one usually quoted.
One source is easy to miss: the ground itself. A path or drive falling back towards the house delivers rainwater to the base of the wall every time it rains. If you have a sunken or ponding driveway, string-line it before accepting any diagnosis, because correcting falls is paving work, not damp proofing.
Cold walls
When it is condensation rather than water from outside
Condensation at the base of a wall behaves the opposite way round to everything above. It is worst in cold still weather, not in rain, and it concentrates where the wall is coldest and the air moves least: behind a wardrobe flat against an outside wall, in the corner of an unheated bedroom. The tell is black spot mould rather than a brown tide line, and a patch that improves when you move the furniture and open the vents.
The mechanism is arithmetic, and BRE’s worked example of autumn condensation lays it out: air at a given temperature and humidity has a dew point, and any surface colder than that gets wet. Nothing has failed. The wall is cold. So the fixes are heating, ventilating and insulating, the same reasoning that explains condensation in a loft. BRE is blunt about how to be sure: logging humidity and temperature in several rooms and outside, every half hour, is the most effective way of diagnosing damp, and a cheap logger out for a fortnight out-argues any single reading.
White deposits
The white powder on the wall: efflorescence, salts or mould
White fluffy deposits on brickwork, render or plaster are almost always soluble salts that were in the materials all along. Water dissolved them, carried them to the surface and evaporated, leaving them behind. That makes efflorescence evidence that water has moved through the wall, not a disease in itself. Brushing it off without finding the water achieves nothing.
Telling the white things apart
Efflorescence powders off cleanly with a dry stiff brush. On new brickwork or fresh render it is normal drying out and stops once the wall has dried, which can take the best part of a year. Do not wash it off with water: that redissolves the salts and carries them back in.
Lime bloom is a hard white crust rather than a powder, leached out of cement or lime and set on the face. A dry brush will not shift it. Cosmetic, and still not a reason to install a damp course. Mould is fuzzy rather than crystalline, often smells, and grows on the surface instead of out of it.
The awkward case is hygroscopic salts, mostly nitrates and chlorides, left by a problem already cured. They pull moisture out of the air, so the wall feels damp on a muggy day with no rain and reads high on a resistance meter. That is the pattern most easily mistaken for rising damp, and it is why BRE names soluble salt analysis on drilled samples as an established diagnostic route. Where the source is long gone, the answer is replastering once the wall is dry.
Do not reach for acid. Acid-based brick cleaners attack some bricks and mortars, and can leave staining worse and more permanent than the efflorescence itself. Find out what the brick is first.
The contested one
Rising damp: real, last on the list, and hard to prove with a hand meter
Rising damp is real. It is one of the four types of damp named in government guidance, defined there as moisture from the ground that rises up through parts of the buildings in contact with the ground.
Anybody telling you it does not exist is overcorrecting, and you can check them in one click. The honest position is narrower: it is the least likely of the causes on this page, and the same guidance says in the same breath that it is usually found in older properties and is often misdiagnosed.
BRE puts it more strongly still: because existing buildings are complicated, dampness is very often misdiagnosed, which leads to more damp, more cost and more disruption later. That is not a neutral outcome. It buys a remedy that does not work and leaves the real defect running.
Which brings up the instrument. The hand-held moisture meter carried on most damp surveys is a resistance meter: it passes a current between two pins and infers wetness from how easily it flows. Salts conduct. So does foil-backed plasterboard, and so do some plasters and paints. BRE’s position is that in skilled hands with regular calibration these meters are a reasonable starting point, but that depending on the building, the materials and its history they can lead to misdiagnosis. A high reading on plaster is a prompt to investigate, not a finding. The government guidance says rising damp can be identified by visual inspection, but that chemical testing is the most appropriate way of confirming it. Nobody should be injecting a wall on the strength of two pins in the skirting.
So if you are holding a survey that recommends injection, three questions are fair and none is rude. What instrument was used, and was anything taken beyond surface readings? What was found outside, at ground level and in the gutters? And what would have to be true for this to be something else?
Then look at the pattern. Ground moisture rises evenly, so it tends to leave a fairly level tide line at a consistent height along a run of wall, in wet and dry weather alike, often with a salt band at the top edge. Blotches at different heights, or damp that stops dead at one downpipe, are telling you something else. Rising damp is what is left when the other four are ruled out, not the first thing to reach for.
Next
What to do next, in order
- Go outside, find the damp proof course line, measure down to the ground and photograph it. Ground at or above that line is your answer.
- Read the water meter, use nothing for half an hour and read it again. That settles whether it is a plumbing leak.
- Stand outside in the next downpour and watch the gutters, downpipes and gullies.
- Keep a fortnight of dated notes: weather, and whether the patch was worse, the same or better.
- Move the furniture off the wall, open the trickle vents and see if it improves.
Only then decide what work is needed. For a lot of readers the honest answer is a spade, a length of downpipe and some patience. Where plaster has blown, replastering over a live source fails every time, so a quote to hack off and re-render before the cause is fixed is a quote to do it twice.
If you rent, this is not only a building problem. The Homes (Fitness for Human Habitation) Act 2018 requires a rented home to be fit to live in, and damp and mould can make it unfit. In the social rented sector, Awaab’s Law gives landlords 10 working days to investigate a potential significant damp and mould hazard, and 24 hours for an emergency one.
Where the cause is the wall itself, that is bricklaying and repointing. Thomas looks at the outside of the wall before he says a word about the inside, and he will tell you when the fix is a spade, not an invoice. You can ask for a written itemised quote or send him a photograph first.
Questions
Common questions
How do I tell rising damp from penetrating damp?
By whether the weather changes it. Penetrating damp darkens within a day of rain and dries back over a dry fortnight. Rising damp does not switch off in dry weather. Condensation is worst in cold still weather and brings black spot mould instead of a tide line. Two weeks of dated notes separate all three.
What is a bridged damp proof course?
The damp proof course is a waterproof layer built into the wall a few courses above the ground to stop ground moisture climbing the brickwork. It is bridged when something lets water step around it: soil, a path or patio laid up to or over it, a raised bed, decking, render carried down past it, or rubble left in the bottom of a cavity.
Does a high moisture meter reading prove rising damp?
No, a hand-held meter of the sort used on damp surveys measures electrical resistance, and salts, foil-backed plasterboard and some plasters all read high whether or not the wall is wet. BRE says these meters can lead to misdiagnosis depending on the building and its history. Government guidance says chemical testing is the most appropriate way of confirming rising damp.
Is white powder on the wall a sign of rising damp?
Not on its own. White fluffy deposits are usually soluble salts that were in the masonry already, carried to the surface by water that then evaporated. They tell you water has moved through the wall, not where it came from. On new brickwork or fresh render it is normal drying out and stops when the wall has dried.
Should I wash efflorescence off the wall?
Not with water: water redissolves the salts and carries them back into the wall, and they come out again the next time it dries. Brush the deposit off dry with a stiff brush, repeat as needed, and let the wall finish drying. Avoid acid-based cleaners, which attack some bricks and mortars.
Read next
Related advice
The guides below pick up where this one leaves off, and the bricklaying and repointing page covers the work itself and the places we cover, if you would rather have the job looked at.
- Water coming in only during driving rain
- Is it a rain leak or a plumbing leak?
- A sunken or ponding driveway: base failure or drainage
- Blown plaster and the hollow sounding wall
Bricklaying and repointing, and where we cover All advice articles
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Been quoted for a damp course and want a second opinion?
Thomas comes out, walks the outside of the wall, checks the ground levels, the gutters and the drains, and puts in writing what he found. If the answer is a spade and a downpipe, he will say so.